The invention relates to a method of mounting a deflection unit around a cathode ray tube.
Cathode ray tubes (CRTs) are used for example, in devices such as television apparatuses and computer monitors.
Positioning, also referred to as xe2x80x98matchingxe2x80x99 the deflection unit and the envelope to each other is done during one of the final stages of manufacture. The deflection unit is positioned on the envelope, more precisely on the neck-cone transition part of the envelope which has all the elements needed for displaying an image on the display device. An image is formed on the display screen of the display device. The position of the deflection unit is varied in respect of the envelope so as to find the best, or at least an acceptable image reproduction, i.e. conforming to pre-set quality specifications, image reproduction, whereafter the positions of the deflection unit and envelope with respect to each other are fixed.
Such matching increases the quality of the image displayed by the display device. There is an ever increasing demand for high-quality image reproduction.
It is an object of the invention to provide a method with which, on average a better image reproduction can be obtained.
To this end, the invention provides a method of mounting a deflection unit around a cathode ray tube as defined by claim 1. The dependent claims define advantageous embodiments.
The inventors have realized that the known methods, in which the axis of the display unit is oriented substantially horizontally, may cause a shift in the position of the deflection unit in respect of the envelope after matching. The weight of the envelope and the deflection unit, especially for the ever increasing sizes of the display devices as currently employed, requires the means for varying the positions of the deflection unit and envelope vis-à-vis each other to counteract the forces of gravity. During matching (sometimes also referred to as alignment), the means therefore exerts a vertically oriented force on the deflection unit. To ensure that the deflection unit is indeed positioned against the envelope, the means often also apply some force in the horizontal direction. The forces by which the position of the deflection unit is actually varied are therefore superimposed on the forces to counteract the forces of gravity. This decreases the accuracy with which these last-mentioned forces may be applied and thereby the accuracy with which the deflection unit and envelope are positioned with respect to each other. Furthermore, once the positions are fixed, the means for varying the positions are released, thereby releasing the mentioned counteracting forces. A recoil of the deflection unit vis-a-vis the envelope may be the result of such a release. The means for fixation may comprise, on the one hand, parts of the deflection units or parts to be attached to the deflection unit and, on the other hand, devices for screwing, soldering, etc. The means for fixing may comprise for example, a clamping band by which one end of the deflection unit is clamped upon the neck portion of the envelope and means for fixing the clamping band, such as means for tightening a screw, and a wedge or wedges to be inserted between the other end of the deflection unit and the envelope, and means for soldering (for example, means for ultrasonic welding the deflection unit to the wedges.
In the method in accordance with the invention the tube axis is oriented vertically, with the neck portion being above the display screen. The neck portion is thus upwardly oriented. The display screen is positioned xe2x80x98face-downxe2x80x99. When, in this position, the deflection unit is positioned on the envelope, the force of gravity does not need to be counteracted by the positioning means. In fact the force of gravity also supplies the force to position the deflection unit against the envelope. In contrast to the known methods, the starting position of the deflection unit at the start of the matching procedure is a stable one. This allows a much better xe2x80x98fine-tuningxe2x80x99 of the position. The forces needed for varying the position are not superimposed on relatively large forces to counteract the force of gravity or to push the deflection unit against the envelope. Preferably, the method in accordance with the invention comprises the step of measuring the image displayed by means of a measuring device located below the display screen, said measuring device supplying its data to a comparison device for comparing the data with standard data, said comparison device supplying data, in dependence on the measured data, to the means for varying the position. Although it is possible, within the invention in its broadest sense, to use human vision to inspect the image formed on the display screen, and regulate the position of the deflection unit, this would require either the use of a camera and inspection of the image on the camera, with a resulting loss of inspection quality, or inspection via a mirror, which would also mean a loss of inspection quality. The alternative, namely direct observation of the image would require the viewer to be positioned xe2x80x98face-upxe2x80x99 under the display screen, which position puts the viewer under great physiological stress and reduces the inspection quality.
Prior to engaging the means for varying the position of the deflection unit in respect of the envelope, the method preferable comprises, the step of positioning the envelope in respect of the recording means by way of envelope-positioning means. Such a (pre-)positioning improves the analysis of the reproduced image. Preferably, the means for positioning the envelope comprise a means for lifting the envelope, which means comprise bearings, preferably ball bearings upon which the envelope rests, and means for moving the envelope in at least two directions perpendicular to the tube axis. Since the tube axis is oriented substantially vertically , the envelope has a tendency to orient its tube axis as much as possible along a preferred, known vertical axis. In prior methods, where the tube axis is oriented substantially horizontally, the envelope will, have a tendency to tilt to an unknown degree unless relatively great counteracting forces are used. Thus, the measure increases the accuracy with which the envelope is oriented vis-a-vis the imaging means. The envelope can be brought into a good position very small and symmetrical forces.